As the world's supply of fossil fuels dwindles, the search for energy resources to compliment and eventually replace fossil fuels has commenced. There are many options, including biofuels, nuclear power, wind power, solar power, and hydrogen. While all of these options are being developed, hydrogen is particularly attractive, especially in terms of environmental impact. Combustion of hydrogen only produces water, not the greenhouse gases that result from the combustion of biofuels or the dangerous wastes that result from nuclear fission. While hydrogen may be derived from natural gas or fossil fuel feedstocks, water is an abundant and obvious hydrogen resource. The technical hurdles that need to be cleared to realize a hydrogen economy include energy efficient production and storage of hydrogen. Hydrogen production issues may involve several technologies, including using vast arrays of solar cells for large scale hydrogen production via electrolysis of water or generating hydrogen via thermolytic cracking of water in nuclear power plants.
The current application focuses on the storage aspect of the problem. On a weight basis, hydrogen contains a great deal of energy, but on a volume basis it is inefficient. In gaseous form, hydrogen may be pressurized, but the tanks needed to maintain the high pressures required to have enough fuel for a hydrogen powered vehicle are prohibitively heavy. Another option is to liquefy the hydrogen, but the temperatures required are very low and use up about ⅓ of the stored energy in the hydrogen just for the refrigeration. Another approach that has been investigated recently is storing hydrogen in complex metal hydrides. The density of hydrogen that can be achieved in these materials is often higher than that observed for either pressurized gas systems or liquid hydrogen systems. Complex metal hydrides have been considered for utility in hydrogen storage applications for many years, but many fail to clear the hurdles presented by reversibility and high gravimetric capacity. La5NiH6 is widely used because of its excellent ability to reversibly absorb and desorb hydrogen, but with a gravimetric capacity of only 1.7 wt % hydrogen, it would be too heavy to use in vehicular applications. Other complex hydrides, such as the alanates, have good hydrogen storage capacities, up to 9 wt. %, but initially were not found to operate under practical conditions with respect to temperature and pressure. A landmark paper by Bogdanovic and Schwickardi (J. Alloys and Comp., 1997, 253-254, 1-9) showed that Ti-doped NaAlH4, could operate reversibly under 200° C. at reasonable pressures with nearly a 5 wt. % hydrogen storage capacity, close to the theoretical maximum of 5.5 wt. % for that material. Chen et al. found that the LiNH2/LiH-based systems could be decomposed in one step to Li2NH or two steps to yield Li3N, yielding 6.5 and 10.4 wt. % hydrogen, respectively (See Chen et al., Nature, 2002, 420, 302). High operating temperatures in excess of 300° C. are required to realize the higher hydrogen capacity. The LiNH2 system has been extended to include MgH2 in several ways, both by looking at reactions based on mixtures of 2 LiNH2 and MgH2 (See Xiong et al., Adv. Mater., 2004, 16, 1522) or via swapping the hydride for the amide in the starting materials, Mg(NH2)2+2 LiH (Leng et al., J. Phys. Chem. B, 2004, 108, 8763; Xiong et al., J. Alloys and Compounds, 2005, 398, 235). These systems operate at lower temperatures than the LiNH2/LiH systems. It is generally thought that the reversible hydrogen storage reactions in this system is:Mg(NH2)2+2LiHLi2Mg(NH)2+2H2 which yields about a 5.5 wt. % theoretical hydrogen storage capacity. Another hydrogen storage system based on 2 LiBH4 and MgH2 has been proposed by Vajo et. al. (see J. Chem Phys. B Letters, 2005, 109, 3719). This system is said to destabilize LiBH4 via the formation of MgB2 in the dehydrided form and reversibly stores up to 10 wt. % hydrogen, but the operating temperature is well over 300° C. Pinkerton et al. reported a new material derived from 2 LiNH2 and LiBH4 which yielded >10 wt. % hydrogen when heated over 250° C., but was not reversible. It was suggested that this material was Li3BH4(NH2)2, but further work has shown that this material is likely Li4BH4(NH2)3 (Pinkerton et al.).
Applicants have developed a ternary system of LiBH4—MgH2—LiNH2 which can reversibly store up to nearly 4 wt. % hydrogen at temperatures below 220° C. These compositions show greater low temperature reversible hydrogen storage compared to binary systems such as MgH2—LiNH2.